The invention relates to voice communication systems and methods and, in particular, to an apparatus and method for minimizing end to end voice delay while transmission delay varies.
xe2x80x9cSource processing delayxe2x80x9d is the time required to convert a source sound to packets. xe2x80x9cTransmission delayxe2x80x9d is the delay between when a packet is sent from a source and when it is received at a destination. xe2x80x9cDestination processing delayxe2x80x9d is the time required to convert packets back into sound. xe2x80x9cVoice delayxe2x80x9d is the sum of source processing delay, transmission delay and destination processing delay.
In ideal digital communication systems, transmission delay is constant and voice packets arrive at regular intervals to allow for continuous reconstructed speech at the destination. However, most digital communication systems do not exhibit a regular arrival of packets due to changes in delay associated with voice packet routing. This variation in transmission delay becomes more problematic in satellite communication systems due to large transmission delays and the large transmission delay fluctuations.
With fixed-voice packet timing, packets are expected at regular intervals. If usable packets do not arrive at the expected time due to an increase in delay or excessive bit errors, there has to be a prediction what the speech might have been, or the voice output has to be muted since there is no data from which to generate speech. In addition, when the packets arrive early, they have to be buffered until they would normally be used. This time the packets wait in the buffer is called buffer delay. Buffer delay increases the voice delay for these packets.
Previous digital systems added enough delay to the system to mitigate the worst case transmission delay variations. Other proposed solutions include adaptively scaling the duration of output speech during both speech and pause segments. Most current terrestrial systems do not have to compensate for large changes in transmission delay and often choose to add small amounts of buffer delay. The only known approaches are those used by GSM (Global Satellite Mobile), USDC (United States Digital Cellular) and JDC (Japan Digital Cellular).
Since these approaches are inadequate for mitigating the effects of large transmission delay variations, there is a significant need for a delay mitigation scheme that maintains a minimum end to end voice delay when voice packet transmission delays fluctuate.